1. Field of The Invention
This invention relates to a light-emitting device and more particularly, to a semiconductor laser device.
2. Description of the Prior Art
One of typical known materials for visible light semiconductor laser devices is (AlGa).sub.0.5 In.sub.0.5 P. When this semiconductor material is used as a semiconductor laser device, it is epitaxially grown on a substrate which is usually made of GaAs. Except for nitrides, (AlGa).sub.0.5 In.sub.0.5 P compounds have the highest band gap energy among those compounds of the elements of Group III-V of the Periodic Table. Accordingly, they are generally used as a semiconductor laser device capable of emitting a laser beam in a red color region. In general, the substrates on which compound semiconductors are deposited by epitaxy include, aside from GaAs, InP, GaP, GaSb, Si and Ge. For the expitaxial growth of (AlGa) InP materials, it has been generally accepted that the substrate should be made of GaAs from the standpoint of the lattice constant matching. Where (AlGa) InP is epitaxially grown on the GaAs substrate, the smallest band gap energy is 1.91 eV for Ga.sub.0.5 In.sub.0.5 P and the largest band gap energy is 2.35 eV for Al.sub.0.5 In.sub.0.5 P.
There is a recent tendency of semiconductor laser devices toward a shorter wavelength of lasing. For realizing the short lasing wavelength, the band gap energy of an active layer, serving as a light-emitting portion, should be great. The active layer should be made of a material of the direct transient type for the generation of a laser beam and should have a hetero structure in order to confine light and a charge current within a narrow region. Moreover, clad layers between which the active layer is sandwiched have a band gap energy which is larger by 0.25 eV or over than that of the active layer.
For the reasons set out above, a short lasing wavelength in the semiconductor laser devices using the GaAs substrate has now been realized by increasing the content of Al in the active layer and the clad layers, thereby increasing the band gap energies.
However, the increase of the band gap energies by the increase in content of Al is limited: the maximum band gap energy is only 2.35 eV for Al.sub.0.5 In.sub.0.5 P wherein the band gap energy of the active layer is 2.10 eV. The lasing wavelength is 590 nm.
Although the increasing content of Al is effective in attaining a short wavelength as stated above, there arises the problem that oxidation takes place in the semiconductor laser device during working operations, resulting in considerable degradation of the characteristics. Thus, the device is not reliable.